Diamond has remarkable characteristics in terms of, for example, strength, thermal conductivity, and chemical resistance, and it is a material with very high potential for applications in mainly industrial materials in which such physical properties can be utilized. CVD diamond has long been known since the discovery of the deposition technique, and it has already been used widely in various industrial application fields, from the uses for cutting and grinding processes to various components including heat dissipating boards (for electronic circuit components), sensors (used in severe environments), optical window materials, detectors (for particle physics experiments), and speaker diaphragms. It is expected that the uses will expand more and more in the future.
The techniques for fabricating diamond for industrial uses can be classified roughly into two techniques, high-pressure synthesis and chemical vapor deposition. The former is the technique in which graphite, for example, serving as the carbon source that is the source material of diamond, is converted into diamond by applying high temperature and high pressure. This technique imitates the generation of diamond in nature. The latter is the technique in which a source material originating from carbon, the constituent element of diamond, is put into a gas state, is then subjected to chemical reactions such as excitation and decomposition with the use of electromagnetic waves or a heating element, and is restructured on a substrate into diamond.
Typical examples of the just-mentioned chemical vapor deposition technique include a plasma CVD (Plasma-assisted Chemical Vapor Deposition) technique, a hot filament CVD (HFCVD: Hot Filament Chemical Vapor Deposition) technique, and a chamber flame method. The differences between these techniques are that these techniques use different means of decomposition and excitation of molecules in vapor phase space, namely, electrons, ions, and radical species in plasma, a heating element, or thermal energy. That is, the ways of imparting energy are different.
The diamond synthesized by the foregoing techniques is formed in a film-like shape, and the diamond is obtained in such a form that the surface shape of the cover material is transferred thereto. By selecting the type of the source material gas, impurities such as boron, phosphorus, and nitrogen can be contained (doped) in the film. The film in which these elements are introduced shows semiconductor-like behaviors electrically, and as the content of the impurity increases, the characteristics of the film change into those of a conductor.
In depositing CVD diamond on various types of carbonaceous substrates, there is a difference in thermal expansion coefficient between the substrate and the CVD diamond layer. If there is an excessive difference in thermal expansion coefficient, the CVD diamond layer is compressed to the substrate or is put under stress in a stretching direction during the cooling process after the deposition and preparation, so the CVD diamond layer may be peeled off.
Conventionally, various techniques to solve this problem have been investigated. In an example, mechanical anchoring of the diamond layer is effected by roughening the substrate surface as a technique of increasing the adhesiveness between the CVD diamond layer and the substrate. For example, Patent Document 1 describes a method of forming a diamond layer on a substrate by a CVD technique, in which a substrate surface is roughened by a blasting process and a film is formed by a CVD technique on the resulting roughened surface. By this method, the film is formed in such a manner that the roughened surface is transferred thereto, whereby the contact area is increased. At the same time, the film is anchored to the substrate in such a manner that wedges are pushed into the film, whereby the adhesiveness between the film and the substrate is increased. The process according to this method can prevent peeling and cracks resulting from the contraction and expansion of the film due to the thermal expansion coefficient difference between the film and the substrate. This process is practiced as a very useful surface treatment technique.